1. Field of the Invention
The present invention is directed to a process for the production of bromine or chlorine from the respective hydrogen halide. The present invention may utilize either anhydrous hydrogen halide or an aqueous mixture which is preferably evaporated and added to the reaction system as a mixture of steam and the hydrogen halide. In the process of the present invention of stoichiometric excess of active nitrogen is maintained within the reaction system over the hydrogen halide introduced to the system. Sulfuric acid is introduced to the top of the reaction system and flows counter currently to the hydrogen halide introduced to the system. Temperature and acid strength are regulated to allow stripping of nitrogen monoxide and nitrogen dioxide in the bottom of the reaction system. Oxygen in the feed gases reacts with nitrogen monoxide, leaving part of the nitrogen monoxide unconverted. At a higher zone in the reaction system the nitrogen dioxide reacts with hydrogen halide and/or nitrosyl-halide and nitrogen monoxide and sulfuric acid to produce halogen gas which is recovered as a gas and nitrosylsulfuric acid which becomes part of the liquid phase stream. In a preferred form of the process hydrogen halide is fed almost at the bottom of the system, thus also stripping nitrogen compounds out of the acid stream, mainly consisting of nitrosyl-halide. Reaction of this nitrosyl-halide with oxygen is of no or little importance in the present process.
2. Prior Art
The following U.S. Patents are related to processes for the production of a halide from the respective hydrogen halide wherein both sulfuric acid and active nitrogen compounds are present: U.S. Pat. Nos. 537,508; 1,930,664; 3,131,028; 3,152,866; 3,201,201; 3,449,079; 3,451,776; 3,544,274.
A process for the production of halogen from the respective hydrogen halide has been commercialized which utilizes the recycle of sulfuric acid, however, the process utilizes catalytic amounts of active nitrogen. The process is set forth in FIG. 1 which is a process flow arrangement wherein gaseous anhydrous hydrogen chloride feed is converted to gaseous chlorine of high purity. Basically, four steps are employed in this process for the conversion of HCl to gaseous chlorine:
Recycle-acid stripping
Oxidation
Absorption-oxidation
Recycle-acid flashing
After conversion, the chlorine gas can be dried and liquefied by standard procedures. Each of the steps employed in this prior art process will be described in further detail hereinafter.
Recycle-Acid Stripping-- In this operation gaseous feed HCl contacts, countercurrently, a hot aqueous sulfuric acid stream of about 80% concentration, which contains the water of reaction and catalyst, taken up in the absorber. The catalyst, mainly present in the form of nitrosylsulfuric acid, is stripped from the acid by the predominant reaction: EQU HNSO.sub.5 + Hcl.fwdarw. NOCl+ H.sub.2 SO.sub.4 ( 1)
in the next lower section of the stripper, oxygen is introduced to strip out the small amount of HCl, which has gone into solution. The acid stream leaves the stripper carrying the water of reaction and only traces of HNSO.sub.5.
Oxidation-- The gas stream from the stripper contains NOCl, excess HCl, oxygen, and a small amount of water, which has been stripped from the hot acid. At the temperature of the stripper this gas reacts slowly, owing to the very limited partial pressure of NO. In the oxidizer, in the absence of the sulfuric acid stream, the gas first is warmed up to increase the rate of NOCl decomposition. Once this reaction kicks off, the highly exothermic oxidative reactions take place and heat has to be removed in order for the reaction to proceed to the desired conversion. Both heat input and heat removal are carried out by heat exchange. The reactions involved can be represented by: EQU 2NOCl= 2NO+ Cl.sub.2 ( 2) EQU 2NO+ O.sub.2 .fwdarw. 2NO.sub.2 ( 3) EQU no.sub.2 + 2hcl.fwdarw. NO+ Cl.sub.2 + H.sub.2 O (4)
for practical purposes, oxygen consumption is limited to the oxidizer, which means that a sufficiently large volume for the oxidation reactions has to be provided to obtain the desired oxygen conversion.
Absorption-Oxidation-- As a result of the oxidation reactions, the amounts of HCl and NOCl are greatly reduced and the reversal of reaction 1 is favored when the oxidizer effluent gas is contacted with the circulating acid stream. NOCl is absorbed in the countercurrent acid-gas contacting, provided in the absorber-oxidizer. There, not only is the catalyst recovered from the gas stream, but also most of the remaining HCl is oxidized to chlorine: EQU NO.sub.2 + 2HCl.fwdarw. NO+ Cl.sub.2 + H.sub.2 O (4) EQU nocl+ H.sub.2 SO.sub.4 .fwdarw. HCl+ HNS0.sub.5 ( 5) EQU no+ no.sub.2 + 2h.sub.2 so.sub.4 .fwdarw. 2hnso.sub.5 + h.sub.2 0 (6)
it has been proven in long duration pilot plant operations, that through the reactions described it is quite feasible to obtain 99+ % conversion of HCl to chlorine with negligible catalyst loss and a relatively low excess of oxygen. Product gas from this operation is cooled and dried with a small stream of cold sulfuric acid. Subsequent chlorine gas treatment depends upon the particular requirements of the chlorine consumer.
Recycle-Acid Flashing-- Sulfuric acid from the stripper bottom contains all of the water of reaction. This is removed by an adiabatic flash to a vacuum set by the stripper bottoms temperature and the desired acid strength.
After flashing, the acid stream is ready for recycling to the absorber-oxidizer. Most of the acid is recycled hot, while a small amount is cooled to provide cooling medium for the product gas at the top of this tower. In effect, the heat of absorption of reaction water vapor supplied the heat of desorption and no added heat supply is required. The heat, removed from the oxidizer and from the cooled acid recycle, is essentially equal to the overall reaction heat of forming chlorine gas.